One more thing ... do you have fresh engine oil and sufficient oil pressure?
Most people don't realize engine oil plays a crucial role in helping to keep the engine at the right temperature.
If none of this identifies the problem, it's likely a key coolant passage in the engine has become blocked. It's not unheard of for engine rebuilders to fail to clean the block completely, leaving junk in the passages that can become dislodge and subsequently block a key passage. (I know, not good news ... especially because this tends to show up after the engine is past the warranty period.)
